Glossary

absolute zero
The lowest possible temperature. The Kelvin temperature scale sets its zero point at absolute zero (-273.15° on the Celsius scale, or -459.4° Fahrenheit).

asteroid
Also called "planetoids" or "minor planets," the asteroids are small rocky bodies that orbit a star (like the Sun). In the Solar System, most asteroids lie between the orbits of Mars and Jupiter. The largest asteroid -- and the first discovered -- is Ceres, about 900 kilometers in diameter. More than 1800 have been catalogued, and probably millions of smaller ones exist.

astronomer
A physicist who studies astronomy. One who is versed in astronomy is one who has knowledge of the laws of the heavenly orbs, or the principles by which their motions are regulated, with their various phenomena.

atom
The building block of matter. The smallest component of matter that retains its chemical properties. An atom consists of a nucleus and at least one electron.

The nucleus of an atom consists of one or more protons and may contain neutrons as well; any electrons surround the nucleus.

aurora
Spectacular array of light in the night sky, caused by charged particles from the Sun hitting the Earth's upper atmosphere. The aurora borealis is seen in the north of the Northern hemisphere; the aurora australis in the south of the Southern hemisphere.

black hole
A region in space where there is an object with such a strong gravitational field that even light cannot escape.

comet
A relatively small body consisting of a frozen mass that travels around the sun in a highly elliptical orbit. This elongated orbit is approaches very near to the sun in its closest point, but recedes to a very great distance from it at its farthest point. A comet commonly consists of three parts: the nucleus, the envelope (or coma), and -- most famously -- the tail.

corona
Outermost atmosphere of the Sun, consisting of hot, low-density gas that extends for millions of miles from the Sun's surface.

Ordinarily the Corona can be seen only during a total solar eclipse when it is visible as a white halo. Its shape varies from almost spherical at sunspot maximum to asymmetrical at minimum. It is very high in temperature.

Doppler, Christian Johann
An Austrian physicist famous for his discovery of the Doppler effect (1803-1853)

Doppler effect
A change in the apparent frequency (or wavelength) of a wave as observer and source move toward or away from each other.

The effect is found with sound waves: as the source and observer move together the apparent frequency of the sound is higher than that produced; as they move apart it is lower. For example, an approaching motorbike -- the pitch of the engine sound appears to drop suddenly as it passes.

A similar effect is observed in electromagnetic radiation (light). The radiation emitted by a source moving rapidly towards the observer will have increased frequency and a smaller wavelength. If the source is moving away from the observer, the observed frequency is reduced, and the wavelength is "stretched." Movement toward the observer results in a blueshift; movement away from the observer results in a redshift. This has great significance in astronomy.

Earth
Third planet from the Sun.

eclipse
The obscuration of the light from a celestial body caused by its passage through the shadow cast by another body. Such an eclipse may be complete (total) or incomplete (partial, annular), depending on the position of the observer.

The obscuration of a planet or star by the moon or a planet, though of the nature of an eclipse, is called an occultation.

The eclipse of a small portion of the Sun by Mercury or Venus is called a transit of the planet.

eclipse, annular
A solar eclipse in which the solar disk is never completely covered but is seen as an annulus or ring at maximum eclipse. An annular eclipse occurs when the apparent disk of the Moon is smaller than that of the Sun.

eclipse, lunar
Caused when the Earth passes between the Sun and the Moon, so that the Earth's shadow falls across the Moon and the Moon is passing through the shadow cast by the Earth.

The lunar eclipse is total when the Moon is passing completely through the Earth's umbra.
The lunar eclipse is partial when the Moon is passing partially through the Earth's umbra at maximum eclipse.
The lunar eclipse can also be penumbral when the Moon is passing only through the Earth's penumbra.

eclipse, solar
Caused when the Moon passes between the Sun and the Earth. This is an eclipse in which the Earth passes through the shadow cast by the Moon.

The solar eclipse is total when the observer is in the Moon's umbra.
The solar eclipse is partial (or Annular) when the observer is in the Moon's penumbra.

ecliptic
The great circle of the celestial sphere, making an angle with the celestial equator of about 23 degrees. This circle is the mean plane of the Earth's orbit around the Sun.

Because the equator of the Earth is at an angle of about 23 degrees to the plane of its orbit, the ecliptic is at the identical angle to the celestial equator, intersecting it at two points: the vernal and autumnal equinoxes.

All of the planets rotate the sun in approximately the same ecliptic.

Lunar ecliptic limit is the space of 12° on the moon's orbit from the node, within which, if the moon happens to be at full, it will be eclipsed.

Solar ecliptic limit is the space of 17° from the lunar node, within which, if a conjunction of the sun and moon occur, the sun will be eclipsed.

electromagnetic field
Field of the electromagnetic force, consisting of electric and magnetic lines of force at each point in space.

electromagnetic radiation
"Waves" of electrical and magnetic "disturbance," radiated as visible light, radio waves, or any other manifestation of the electromagnetic spectrum. The distance between successive crests of each wave is known as the wavelength, and varies considerably among electromagnetic forms.

Radio waves, infrared radiation, visible light, ultraviolet radiation, x-rays, and gamma rays all are electromagnetic radiations. In a vacuum, all electromagnetic radiation travels at the speed of light. The shorter the wavelength of a radiation, the more energetic it is.

electromagnetic spectrum
The complete range of electromagnetic radiations, from very short-wavelength (high-frequency) gamma-rays, through X-rays and ultraviolet light to the small range of visible light, and further to infrared radiation, microwave, and the comparatively long-wavelength low-frequency radio waves.

electromagnetic wave
A pattern of electric and magnetic fields that moves through space. Depending on the wavelength, an electromagnetic wave can be a radio wave, a microwave, an infrared wave, a wave of visible light, an ultraviolet wave, a beam of X rays, or a beam of gamma rays.

electromagnetism
Magnetism produced by an electric current; electromagnetism was discovered when it was observed that a copper wire carrying an electric current can magnetize pieces of iron or steel near it. It was concluded that electrical and magnetic forces are intimately related, since a changing electric field produces a magnetic field, and vice versa.

electron
A subatomic, negatively charged particle that appears in every neutral atom, surrounding the positively charged nucleus, like bees around honey.

The combined charge of the orbiting electrons is balanced (in a neutral atom) by the charge of an equal number of positively charged protons in the atomic nucleus.

hydrogen
A gaseous element that is normally colorless, odorless and highly flammable; the simplest, lightest, and most abundant element in the universe.

galaxy
A huge collection of millions, billions, or trillions of stars, plus interstellar gas and dust, held together by their mutual gravity.

gravity
Gravity is the force of attraction between all masses in the universe; especially the attraction of the earth's mass for bodies near its surface; "gravitation cannot be held responsible for people falling in love" -- Albert Einstein

helium
An element which, after hydrogen, is the second-lightest and second-most-abundant in the Universe.

infrared
Electromagnetic waves in the frequency range just below visible light, corresponding to radiated heat. The wavelengths are longer than visible light but shorter than radio waves; lying outside the visible spectrum at its red end.

magnetic field
See electromagnetic field.

mass
Mass and weight are often used, in a general way, as interchangeable terms. But the two ideas, mass and weight, are quite distinct. Mass is the quantity of matter in a body; weight is the effect that gravity has on that matter. A mass of sugar and a mass of lead are assumed to be equal when they show an equal weight by balancing each other in the scales.

meteor
A streak of light in the sky at night that results when a mass of stone or other substances hits the Earth's atmosphere and air friction causes it to melt or vaporize or explode.

A "shooting star" -- the streak of light in the sky produced by the transit of a meteoroid through the Earth's atmosphere; also the glowing meteoroid itself. The term "fireball" is sometimes used for a meteor approaching the brightness of Venus; the term "bolide" for one approaching the brightness of the full Moon.

meteoroid
A small body moving through space, or revolving about the sun, which would break up and be seen as a meteor on entering the Earth's atmosphere.

methane
A light, colorless, inflammable gas. It is a hydrogen compound in which every carbon atom is surrounded by four hydrogen atoms.

Milky Way Galaxy
The galaxy that contains our Sun. The term Milky Way also refers to the luminous tract, or belt, stretching across the heavens in a dark sky, which is the central disk of the galaxy.

moon
A secondary planet, or satellite, revolving about any member of the solar system; as, the moons of Jupiter or Saturn.

Moon
The celestial orb which revolves round the Earth; it is the satellite of the Earth; a secondary planet, whose light, borrowed from the Sun, is reflected to the Earth, and serves to dispel the darkness of night.

Moon, full
The Moon with its whole disk illuminated, as when it is opposite to the Sun.

Moon, new
The Moon in its first quarter, or when it first appears after being invisible. It is the day when the new moon is first seen or the first day of the lunar month.

Moonrise, Moonset
The times at which the apparent upper limb of the Moon is on the astronomical horizon.

neutron
An elementary particle with zero charge and mass about equal to a proton; enters into the structure of the atomic nucleus

neutron star
A star that has collapsed under its own gravity, with vast gravitational and magnetic forces. It is called a neutron star because with that much gravity, protons fuse with electrons to form neutrons, so the star is almost entirely composed of neutrons.

orbit
The path in space that a celestial body follows during its periodical revolution around another body; as,the orbit of the Earth around the Sun, of the Moon around the Earth.

penumbra
Less than full shadow (umbra).

The portion of a shadow in which light from an extended source is partially but not completely cut off by an intervening body; the area of partial shadow surrounding the area of full shadow (umbra)

"The shadow cast, in an eclipse, where the light is partly, but not wholly, cut off by the intervening body; the space of partial illumination between the umbra, or perfect shadow, on all sides, and the full light." -- Sir Isaac Newton

planet
An object that is formed in the disk surrounding a star. To be called a planet, an object must be more massive than Pluto (1/500 the Earth's mass) and less massive than ten times Jupiter's mass. Unlike stars, planets do not produce light of their own but merely reflect that of the star(s) they orbit.

proton
A subatomic particle with positive electric charge which can only be found in the nucleus of the atom. Every atom has at least one proton in its nucleus; the number of protons determines the element. The hydrogen atom has a single proton in its nucleus.

star
From a root meaning, to scatter, and originally applied to the stars as being strewn over the sky, or as being scatterers or spreaders of light.

A star is one of the innumerable luminous bodies seen in the heavens.

Solar System
The Sun, with the group of celestial bodies which, held by the sunŐs attraction, revolve around it. The system includes the major planets, with their satellites; the minor planets, asteroids, comets, and meteoroids.

solar flare
Sudden and dramatic release of a huge burst of solar energy through a break in the Sun's chromosphere in the region of a sunspot. Effects on Earth include aurorae, magnetic storms, and radio interference.

solar wind
A Stream of energetic charged particles flowing from the Sun at a speed of about 600 km sec-1. It is the effects of the solar wind that produce aurorae in the Earth's upper atmosphere, that cause the tails of comets to stream back from the Sun, and that distort the symmetry of planetary magnetospheres.

Sun
The star that the Earth orbits.

The luminous orb, the light of which constitutes day, and its absence night. It is the central body around which the earth and planets revolve, by which they are held in their orbits, and from which they receive light and heat.

The Sun's luminous surface is called the photosphere, above which is an envelope consisting partly of hydrogen, called the chromosphere, which can be seen at the time of a total solar eclipse. Above the chromosphere, and sometimes extending out millions of miles, are luminous rays or streams of light which are visible only at the time of a total eclipse, forming the solar corona.

Sunrise/Sunset
The times at which the apparent upper limb of the Sun is on the astronomical horizon.

Sunspot
A cooler, darker spot appearing periodically on the surface of the sun. It is associated with a strong magnetic field. A sunspot represents a comparatively cool depression. Sunspots occur in cycles of about 11 Earth-years in period and their individual duration is a matter of Earth-days only. Sunspots usually occur in pairs of opposite polarity about 30° N and S of the equator, and move in unison across the face of the Sun towards the solar equator.

telescope
An optical instrument used in viewing distant objects. A telescope assists the eye chiefly in two ways; first, by enlarging the visual angle under which a distant object is seen, and thus magnifying that object; and, secondly, by collecting, and conveying to the eye, a larger beam of light than would enter the naked organ, thus rendering objects distinct and visible which would otherwise be indistinct and or invisible.

umbra
The portion of a shadow cone in which none of the light from an extended light source can be observed.

The conical shadow projected from a planet or satellite, on the side opposite to the sun, within which a spectator could see no portion of the sun's disk. Used in ontradistinction from penumbra. See also penumbra.

Universe
All created things viewed as constituting one system or the whole body of things, or of phenomena. It is the total celestial cosmos!

wane
The apparent decrease in the illuminated part of the moon to the eye of a spectator.

wavelength
The distance (measured in the direction of propagation) between two points in the same phase in consecutive cycles of a wave. For example: The distance between two successive peaks (often called crests) of the propagating disturbance of a wave.

white dwarf
A small, faint, dense, dying star that has used up its nuclear fuel and is slowly fading from view.


The text in this glossary has been research with two primary sources:

  1. http://nedwww.ipac.caltech.edu/level5/Glossary/frames.html
  2. http://smac.ucsd.edu/cgi-bin/http_webster
For more in depth information please refer to LEVEL5 Glossary. "LEVEL5 is a hyperlinked document, providing multiple paths to information, presented at a variety of levels of complexity: from simple definitions up to essays, recent research articles, detailed monographs, and extensive reviews."

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